Method for making ultralow platinum loading and high durability membrane electrode assembly for polymer electrolyte membrane fuel cells

ABSTRACT

A method of making a catalyst layer of a membrane electrode assembly (MEA) for a polymer electrolyte membrane fuel cell includes the step of preparing a porous buckypaper layer comprising at least one selected from the group consisting of carbon nanofibers and carbon nanotubes. Platinum group metal nanoparticles are deposited in a liquid solution on an outer surface of the buckypaper to create a platinum group metal nanoparticle buckypaper. A proton conducting electrolyte is deposited on the platinum group metal nanoparticles by electrophoretic deposition to create a proton-conducting layer on the an outer surface of the platinum nanoparticles. An additional proton-conducting layer is deposited by contacting the platinum group metal nanoparticle buckypaper with a liquid proton-conducting composition in a solvent. The platinum group metal nanoparticle buckypaper is dried to remove the solvent. A membrane electrode assembly for a polymer electrolyte membrane fuel cell is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Pat. ApplicationNo. 16/560,718 filed on Sep. 4, 2019, which claims priority to U.S.Provisional Application No. 62/739,432 filed on Oct. 1, 2018, entitled“A METHOD FOR MAKING ULTRALOW PLATINUM LOADING AND HIGH DURABILITYMEMBRANE ELECTRODE ASSEMBLY FOR PEMFCS”, the disclosures of which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to polymer electrolyte membranefuel cells, and more particularly to catalyst layers for polymerelectrolyte membrane fuel cells.

BACKGROUND OF THE INVENTION

The most critical issues hindering the commercialization of polymerelectrolyte membrane fuel cells (PEMFCs) are the high cost, especiallycost of the precious metal, relative low performance at low Pt loading,and poor long-term durability. The cathode catalyst layers of currentPEMFCs consist of three phases: a matrix of carbon black grainsproviding the electronic conductivity; nano-size platinum (Pt) particlessupported on carbon black as catalyst; and an electrolyte network ofperfluorosulfonate ionomer to ensure proton conductivity. The mainsource of voltage loss occurs due to the poor kinetics of the oxygenreduction at low temperatures, followed by the ohmic losses of protonand electronic transport in the catalyst layer due to inadequatecontacts among carbon particles and ill-defined and ill-controlledconducting paths. Serious problems arise from the oxygen transport athigher current densities due to the flooding of pores in the catalystlayer from the build-up of water inside and are caused by a lack ofpredesigned micro-scale water management capability within the layer.Poor oxygen reduction reaction (ORR) kinetics creates a need for high Ptloadings and results in high manufacturing costs. There are numerousmicro-pores in the carbon black that can trap Pt nanoparticles,resulting in a failure of establishing the triple phase boundary (TPB)condition among mass transport, proton conductive, and electronconductive. This fraction of Pt is therefore not utilized since theelectrochemical reactions could not occur at these sites, which cause areduction in the overall Pt utilization. Additionally, carbon black canbe corroded under the severe conditions in the cathode, resulting in lowcell stability and short service life.

One of the major cost contributors to PEMFCs systems for automotive andstationary power applications is the platinum group metal (PGM) cathodeelectrocatalyst. This high cost is due to the high catalyst loadingnecessary to overcome the limitations of low ORR activity, low PGMutilization within the electrode layer, and loss of ORR activity withoperating time. The United States Department of Energy 2020 goals forPGM loading and stack durability are ≤0.125 g PGM/kW and ≥5,000operating hours, respectively. The current durability status ofautomotive fuel cells, as demonstrated in the DOE-EERE TechnologyValidation program, is >2,500 hours for stacks with high cathodecatalyst loadings and stack costs far exceeding the DOE targets, evenwhen projected to high volume manufacturing.

Recently, carbon nanotube (CNT) and carbon nanofiber (CNF) andcombinations of these materials have been explored as catalyst supportsin PEMFCs because of their unique properties, e.g. high conductivity ofabout 10⁴ S/cm, and large specific surface areas of up to 1300 m²/g.Additionally, CNTs/CNFs have little micro-porosity and excellentresistance to electrochemical corrosion. The conventional ink process tomake the cathode catalyst layer (CCL) largely suppresses Pt ORRutilization (<50%). Buckypaper is a completely new support concept andits structure not only maximizes Pt ORR utilization (>90%), but alsoimproves PEMFC water management to prevent flooding. In addition, thebuckypaper can also be used to support Pt core-shell, Pt alloys, and Ptnano-structure catalysts with high intrinsic ORR activities, whichfurther gives buckypaper a significant potential for ultralow Pt PEMFCs.Carbon nanotube and nanofiber film-based membrane electrode assembliesare described in Zheng et al US 8,415,012 (Apr. 9, 2013), the disclosureof which is incorporated fully by reference.

SUMMARY OF THE INVENTION

A method of making a catalyst layer of a membrane electrode assembly(MEA) for a polymer electrolyte membrane fuel cell includes the step ofpreparing a porous buckypaper layer comprising at least one selectedfrom the group consisting of carbon nanofibers and carbon nanotubes.Platinum group metal nanoparticles are deposited in a liquid solution onan outer surface of the buckypaper to create a platinum nanoparticlebuckypaper. A proton conducting electrolyte is deposited on the platinumnanoparticles by electrophoretic deposition to create aproton-conducting layer on an outer surface of the platinumnanoparticles. An additional proton-conducting layer is deposited bycontacting the platinum nanoparticle buckypaper with a liquidproton-conducting composition in a solvent. The platinum nanoparticlebuckypaper is dried to remove the solvent.

The step of contacting the platinum nanoparticle buckypaper with aliquid proton-conducting composition in a solvent can comprise at leastone selected from the group consisting of the liquid drop method and theliquid dipping method.

The proton-conducting electrolyte can include at least one selected fromthe group consisting of Nafion, polyvinylidene fluoride (PVDF)/Nafioncomposite, and Nafion/silica composite. The proton-conducting layer canbe from 2-10 wt %, based on the total weight of the catalyst layer. Theproton conductivity of the proton-conducting layer can be from 0.01-0.2Siemens/cm.

The buckypaper can have a porosity of from 50% to 90% before thedeposition of the platinum group metal nanoparticles and theproton-conducting layer. The buckypaper layer can have a graduatedporosity, with the porosity being less on a side of the buckypaper layerto abut a proton exchange membrane of the membrane electrode assembly.The porosity of the buckypaper layer can be graduated from a maximumporosity difference of 40% to a minimum porosity difference of 10%. Thebuckypaper can have less than 1% binder, based on the total weight ofthe buckypaper layer.

The platinum group metal nanoparticles can be depositedelectrochemically. The platinum group metal nanoparticles can have adimension of from 2 to 10 nm. The platinum group metal nanoparticles canbe from 30 to 80% wt %, based on the total weight of the catalyst layer.The platinum group metal (PGM) nanoparticles can include at least oneselected from the group consisting of platinum, platinum nickel alloy,platinum copper alloy, platinum cobalt alloy, platinum iron alloy,platinum iridium alloy, and platinum palladium alloy. The platinum groupmetal nanoparticles can be core-shell structures including a platinumshell and a core comprising at least one selected from the groupconsisting of nickel, copper, cobalt, iron, iridium, and palladium.

A membrane electrode assembly (MEA) for polymer electrolyte membranefuel cells (PEMFCs), can include a catalyst layer. The catalyst layercan include a porous buckypaper layer comprising at least one selectedfrom the group consisting of carbon nanofibers and carbon nanotubes, thebuckypaper having an outer surface. Platinum group metal nanoparticlesare provided on the outer surface of the buckypaper, and have outersurfaces. A proton-conducting electrolyte layer is deposited on outersurfaces of the buckypaper and the platinum group metal nanoparticles.The proton conducting layer comprises a first, electrophoreticallydeposited proton-conducting layer on the platinum group metalnanoparticles, and a second proton-conducting layer deposited by aliquid contact method on the Pt particles and the buckypaper.

The membrane electrode assembly can further include a proton exchangemembrane. The membrane electrode assembly can include a second catalystlayer. The proton exchange membrane can be positioned between thecatalyst layers. The membrane electrode assembly can further include twoelectrically conductive and porous gas diffusion layers (GDL) connectedto the surface of two catalyst layers.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the invention is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 is a schematic diagram, partially enlarged, of a polymerelectrolyte membrane fuel cell according to the invention.

FIG. 2 is a schematic diagram of a catalyst layer with Pt particlesdeposited on a buckypaper substrate.

FIG. 3 is a schematic diagram of the catalyst layer of FIG. 2 , with anelectrophoretically deposited proton-conducting layer on the Ptparticles.

FIG. 4 is a schematic diagram of the catalyst layer of FIG. 3 , with aproton-conducting layer deposited by a liquid contract method on the Ptparticles with the electrophoretically deposited proton conductinglayer, and also on the buckypaper.

FIG. 5A is a schematic diagram of carbon nanotubes and nanofibers; FIG.5B is an SEM of buckypaper; FIG. 5C is an SEM of Pt deposited onbuckypaper; FIG. 5D is an SEM of a proton-conducting layer deposited ona Pt-loaded buckypaper.

FIG. 6A is an SEM image of a cross-section of a two-layer buckypaper;the left side of the buckypaper is made of a mixture of CNTs and CNFs,while the right side is made of CNFs; FIG. 6B is a graph showing Ptdistribution along the cross-section of the buckypaper obtained by EDSanalysis; FIG. 6C is an SEM image of the surface of the CNT/CNFsublayer; and FIG. 6D is an SEM image of the CNF sublayer.

FIG. 7 is a plot of the cell potential and power density as a functionof the current density for a 50 cm² MEA operating with oxygen.

FIG. 8 is a plot of the cell potential and power density as a functionof the current density for a 50 cm² MEA operating with air.

DETAILED DESCRIPTION OF THE INVENTION

A method of making a catalyst layer of a membrane electrode assembly(MEA) for a polymer electrolyte membrane fuel cell includes the step ofpreparing a porous buckypaper layer comprising at least one selectedfrom the group consisting of carbon nanofibers and carbon nanotubes.Platinum group metal (PGM) nanoparticles are deposited in a liquidsolution on an outer surface of the buckypaper to create a PGMnanoparticle buckypaper. The advantages of using electrochemicaldeposition of the PGM are that the PGM nanoparticles can be depositednon-uniformly on the surface of CNTs and CNFs. The PGM nanoparticles areelectrodeposited on the most accessible sites on the buckypaper and arenot covered by either CNTs or CNFs. The electrodeposition also ensuresthat the PGM nanoparticles are located on the electron pathway. The PGMnanoparticle particle shape and size can be controlled in order toenhance its intrinsic ORR activity and stability. The PGM nanoparticlesare typically grown in aqueous solution with salts (such as H₂PtCl₆)with low voltage pulses with less than 1 V. The particle size can becontrolled by the pulse width, pulse duty cycle, and total pulsenumbers.

A proton conducting electrolyte is deposited on the PGM nanoparticles byelectrophoretic deposition to create a proton-conducting layer on anouter surface of the PGM nanoparticles. The electrophoretic depositionis the process applied to colloidal particles suspended in a liquidmedium migrate under the influence of an electric field. Typically, highvoltage between 100-500 V is used during electrophoretic deposition.

An additional proton-conducting layer is then deposited by contactingthe PGM nanoparticle buckypaper with a liquid proton-conductingcomposition in a solvent. The PGM nanoparticle buckypaper is dried toremove the solvent. The step of contacting the PGM nanoparticlebuckypaper with a liquid proton-conducting composition in a solvent caninclude at least one selected from the group consisting of the liquiddropping method and the liquid dipping method.

The proton-conducting electrolyte can be any suitable such material. Theproton-conducting electrolyte can include at least one selected from thegroup consisting of Nafion (Chemours Company, Wilmington Delaware),polyvinylidene fluoride (PVDF)/Nafion composite, and Nafion/silicacomposite. Other proton-conducting electrolytes are possible. The protonconductivity of the proton-conducting layer can be from 0.01-0.2Siemens/cm. The proton conductivity of the proton conducting layer canbe 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11,0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, and 0.2 Siemens/cm, orwithin a range of any high value and low value selected from thesevalues.

The total weight of the proton-conducting layers can be from 2-10 wt %,based on the total weight of the catalyst layer. The proton-conductinglayer can be 2, 3, 4, 5, 6, 7, 8, 9, and 10 wt. % based on the totalweight of the catalyst layer, or can be within a range of any high valueand low value selected from these values.

The buckypaper can have a porosity of from 50% to 90% before thedeposition of the PGM nanoparticles and the proton-conducting layer. Thebuckypaper porosity before deposition of the PGM nanoparticles and theproton-conducting layers can be 50, 51, 52, 53, 54, 55, 56, 57, 58, 59,60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77,78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, and 90 %, or can bewithin a range of any high value and low value selected from thesevalues.

The buckypaper layer can have a graduated porosity, with the porositybeing less on a side of the buckypaper layer to abut a proton exchangemembrane of the membrane electrode assembly. The porosity of thebuckypaper layer can be graduated from a maximum porosity difference,the difference between the porosity on the high side and the porosity onthe low side, of 40% to a minimum porosity difference of 10%. Theporosity difference can be 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38,39, and 40 %, or within a range of any high value and low value selectedfrom these values.

The platinum group metal nanoparticles can be comprised of Iridium,(Ir), Osmium (Os), Palladium (Pd), Platinum (Pt), Rhodium (Rh) andRuthenium (Ru), and mixtures thereof. Platinum is a commonly used PGMmaterial. The PGM nanoparticles can be deposited electrochemically. ThePGM nanoparticles can have a dimension of from 2 to 10 nm. The PGMnanoparticles can have a maximum dimension of 2, 3, 4, 5, 6, 7, 8, 9,and 10 nm, or within a range of any high value and low value selectedfrom these values. The PGM nanoparticles can be from 30 to 80% wt %,based on the total weight of the catalyst layer. The PGM nanoparticlescan be 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45,46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, and 80wt. %, based on the total weight of the catalyst layer, or can be withina range of any high value and low value selected from these values.

The platinum group metal (PGM) nanoparticles can include alloys ofplatinum group metals. For example, the PGM nanoparticles can compriseat least one selected from the group consisting of platinum nickelalloy, platinum copper alloy, platinum cobalt alloy, platinum ironalloy, platinum iridium alloy, and platinum palladium alloy. Otheralloys are possible. The PGM nanoparticles can also be core-shellstructures including a platinum shell and a core comprising at least oneselected from the group consisting of nickel, copper, cobalt, iron,iridium, and palladium.

A membrane electrode assembly (MEA) 10 for polymer electrolyte membranefuel cells (PEMFCs) is shown in FIG. 1 . The MEA 10 can include a protonexchange membrane 14, catalyst layers 18 and 22 on sides of the protonexchange membrane 14, and gas diffusion layers 26 and 30. The catalystlayers 18 and 22 comprise a porous buckypaper layer 34 comprising atleast one selected from the group consisting of carbon nanofibers andcarbon nanotubes. The buckypaper 34 has an outer surface. PGMnanoparticles 40 are deposited on the outer surface of the buckypaper34, as shown in FIG. 2 .

Proton conducting electrolyte layers are deposited on outer surfaces ofthe buckypaper 34 and the PGM nanoparticles 40. A first,electrophoretically deposited proton-conducting layer 44 is depositedelectrophoretically on the PGM nanoparticles, as shown in FIG. 3 . Theelectrophoretic deposition process is a charge exchange depositionprocess that favors deposition on the PGM nanoparticles over the carbonof the buckypaper, because the PGM nanoparticles and the protonconducting electrolyte are electronically more conductive. A secondproton conducting layer 50 is deposited by a liquid contact method onthe PGM nanoparticles 40, on the first electrophoretically depositedproton conducting layer 44 and also on the buckypaper 34, as shown inFIG. 4 . There are shown and described a single electrophoreticallydeposited proton conducting layer 44 and liquid contact deposited protonconducting layer 50, however, multiple such layers could be deposited.Also, the proton conducting material that is deposited by anelectrophoretic deposition process and by the liquid contact process canbe the same proton conducting material, or can be different protonconducting materials. The construction and materials of the catalystlayer 18 on one side of the proton exchange membrane 14 can be the sameor different from the materials and construction of the other catalystlayer 22. The catalyst layer of the invention can be the cathodecatalyst layer.

The proton exchange membrane 14 can be constructed in known fashion. Theproton conducting material used to form the proton exchange membrane 14can be the same proton conducting material used to coat the PGMnanoparticles 40 in the electrophoretic deposition process, or depositedon the PGM nanoparticles and the buckypaper 34 by the liquid contactprocess, or the proton conducting material used to form the protonexchange membrane can be a different material. A common material used inproton exchange membranes is Nafion.

The electrically conductive and porous gas diffusion layers (GDL) 26 and30 can be connected to surfaces of the two catalyst layers 18 and 22,respectively. The gas diffusion layers can have any suitableconstruction, and can be made from known materials. The gas diffusionlayer can be a single layer or multiple layers. A single layer of theGDL can be made with carbon fibers, carbon tubes, or their mixture.Multiple layers of GDL can be made based on a single layer of GDL, withan added a layer of carbon black. The layer of carbon black can be amicro porous layer and can provide a lower porosity than a GDL withoutthe carbon black layer.

This invention provides an improved membrane electrode assembly (MEA),in which the most of platinum group metal (PGM) catalytic particles arelocated at sites that satisfy the triple-phase boundary (TPB) conditionand maximize the PGM usage. The catalytic layer can be made withfree-standing buckypaper consisting of carbon nanotubes (CNTs) and/orcarbon nanofibers (CNFs) as support. The buckypaper can be constructedusing the filtering method without any binder. The unique microstructureand well-connected nanotube network ensures a high electronconductivity. The PGM nanoparticles are deposited electrochemically in aliquid solution on the outermost surface area of an established porousCNT/CNF buckypaper network such that the locations of thesenanoparticles are accessible by both electrons and gas. The surfaces ofthe deposited PGM nanoparticles and buckypaper network are coated in alayer of Nafion electrolyte using electrophoretic deposition (EPD) in aNafion monomer solution and combined with a liquid contact method suchas the liquid dropping or liquid dipping method, in order for the PGMnanoparticles to be accessible by protons. One major advantage of usingbuckypaper as support is that the platinum (Pt)/buckypaper cathode showsgreater durability in electrochemical oxidation than the Pt/carboncathode, which is due to the higher corrosion resistance of buckypaperbecause of its high graphitization degree. The high porosity ofbuckypaper also improves the mass transfer process within the catalystlayer resulting in better Pt utilization.

The buckypaper microstructure can be tailored to a gradient-structure sothat PGM nanoparticles have higher density at the membrane and lowerdensity at the air side to match the proton distribution in thecatalytic layer. The porosity of the buckypaper can be controlled to behigher at the air side and lower at the membrane side to optimize themass transfer in the catalytic layer. This will help to avoid “flooding”related issues.

The invention is based on the concept that that the activity, stability,utilization, and high current density performance of low PGM,PGM-alloy-nanoparticle-based cathode electrocatalyst layers can begreatly enhanced by controlling the distribution of the PGM alloynanoparticles and ionomer, and by introducing highly graphiticstructured carbon nanotube supports with a porosity gradient.Introducing PGM alloy enhances its intrinsic ORR activity. Controllingthe distribution of PGM alloy nanoparticles and ionomer improve catalystutilization. Using a highly-graphitic structured carbon support enhancesreactant access and improve support stability against oxidation.

The buckypaper has a unique microstructure of well-connected nanotubesand nanofibers that ensures a high conductivity pathway for electrons.The buckypaper is mechanically and chemically stabile without anybinder, and the specific surface area and porosity of the buckypaper canbe controlled by changing the ratio between the number of MWMTs and CNFsas well as the diameters of MWMTs and CNFs.

The membrane electrode assembly (MEA) for a fuel cell can include acatalyst layer comprising a plurality of catalyst nanoparticles disposedon buckypaper. The catalyst layer can include 1 wt-% or less binderbased on the total weight of the catalyst layer following deposition ofthe catalyst nanoparticles. The catalyst layer can include 0.5 wt-% orless binder, or 0.25 wt-% or less binder, or 0.1 wt-% or less binder, or0.05 wt-% or less binder, or the catalyst layer can include no binderfollowing deposition of the catalyst nanoparticles.

As used herein, “binder” is used to refer to compounds and compositionsused to create adherence between the nanofilaments forming thebuckypaper that are added during the formation of the buckypaper.Exemplary binders include perfluorinated polymers, such as those sold byE. I. Du Pont De Nemours and Company under the TEFLON mark, andperfluorinated, sulfonic acid resins, such as those sold by E. I. DuPont De Nemours and Company under the NAFION mark.

As used herein, “buckypaper” is used to refer to a film-like, stablecomposite comprising a web of single-wall carbon nanotubes, multi-wallcarbon nanotubes, carbon nanofibers, or a combination thereof. In theembodiments disclosed herein, the buckypaper can be stabilized largelyby entanglement of flexible single-wall nanotubes and small diametermulti-wall nanotubes around larger, more rigid nanofibers and the largediameter multi-wall nanotubes.

As used herein, the terms “carbon nanotube” and the shorthand “nanotube”refer to carbon fullerene structures having a generally cylindricalshape and typically having a molecular weight ranging from about 840 togreater than 10 million Daltons. Carbon nanotubes are commerciallyavailable, for example, from Carbon Nanotechnologies, Inc. (Houston,Tex. USA), or can be made using techniques known in the art. As usedherein, the term “small diameter MWNT” refers to multiwall nanotubeshaving a diameter of 10 nm or less, and the term “large diameter MWNT”refers to multiwall nanotubes having a diameter of more than 10 nm. Theterm “large diameter CNF” refers to carbon nanofibers having a diameterof 10 nm or more. As used herein, the terms “carbon nanofilament” and“nanofilament” are used interchangeably to describe single-wall carbonnanotubes, multi-wall carbon nanotubes and carbon nanofibers.

Single-wall nanotubes can have a diameter of less than 5 nanometers anda length between 100-1000 nanometers. Multi-wall nanotubes aremulti-wall nanotube structures and can have a diameter ranging from lessthan 10 nanometers to 100 nanometers and a length between 500 nanometersand 500 micrometers. Carbon nanofibers can have a diameter from 100nanometers to 200 nanometers and a length from 30 to 100 micrometers.

The buckypaper can include at least two types of nanofilaments selectedfrom single-wall nanotubes, small diameter multi-wall carbon nanotubes,large diameter multi-wall carbon nanotubes, and carbon nanofibers. Thebuckypaper used in the catalyst layer can include (a) single-wallnanotubes, 55 small diameter multi-wall nanotubes, or both, and (b)large diameter multi-wall nanotubes, carbon nanofibers, or both. Theratio of the nanofilaments of (a) to the nanofilaments of (b) can rangefrom 1:2 to 1:20. In some embodiments, the ratio of (a) to (b) can rangefrom 1:2 to 1:15, or 1:2.25 to 1:8, or from 1:2.5 to 1:6.

The buckypaper can include at least a first layer and a second layer.The first and second layers can be the same or different. The firstlayer can include (a) single-wall nanotubes, small diameter multi-wallnanotubes, or both, and (b) large diameter multi-wall nanotubes, carbonnanofibers, or both, and the second layer can include multi-wallnanotubes carbon nanofibers, or both.

The buckypaper microstructure can be tailored by adjusting the startingmaterials and nanotube dispersion to achieve a target porosity, poresize, surface area and electrical conductivity. The catalyst layer canbe formed by depositing a plurality of catalyst nanoparticles on thebuckypaper after the buckypaper has been formed. The buckypaper can beformed using less than 1 wt-% binder, or any smaller amount disclosedherein. By depositing the catalyst nanoparticles after the buckypaper isformed with minimal binder, the catalyst nanoparticles can be directlydeposited at the most efficient sites directly on the buckypaper formaximizing the three phase reaction coefficient. Relative toconventional MEAs, the MEA according to the design disclosed herein hasa higher catalyst utilization efficiency at the electrodes, a higherpower output, and better resistance to oxidation, as well as longerservice life.

The buckypaper can be fabricated using the steps of (1) dispersing anamount of MWNT’s, CNFs, or both MWNTs and CNFs, with an amount of SWNTsin a liquid to form a 15 suspension (wherein the nanotubes separate intoindividual fibers or small bundles and float in the non-solvent due tothe large surface area of the nanotubes and strong molecularinteractions); and then (2) filtering the suspension to remove theliquid, to yield a film that includes MWNTs, CNFs, or 20 both MWNTs andCNFs, with SWNTs interspersed there-through. In another embodiment, step(2) utilizes vaporization of the liquid to remove the liquid and formthe buckypaper. It is also possible to use a combination of filtrationand evaporation, either sequentially or simultaneously. The vaporizationor filtration process may include the addition of heat, a pressurereduction, or a combination thereof.

The liquid can be a non-solvent. As used herein, the term “non-solvent”refers to any liquid media that are essentially non-reactive with thenanotubes and in which the nanotubes are virtually insoluble. Examplesof suitable non-solvent liquid media include water and volatile organicliquids, such as acetone, ethanol, methanol, and n-hexane. The liquidmay be an aqueous solution, and may be an aqueous-organic liquidmixture. Low-boiling point nonsolvents are typically preferred so thatthe non-solvent can be easily and quickly removed from the matrixmaterial. The liquid optionally may include a surfactant (such as anon-ionic surfactant, e.g., Triton X-100, Fisher Scientific Company, NJ)to enhance dispersion and suspension stabilization. The surfactant canbe removed along with the rest of the liquid in the filtration orvolatilization step.

Example

Multiwall carbon nanotube (MWNT) and carbon nanofibers (CNF) as shown inFIG. 5A were mixed and dispersed in a solution of dimethylformamide(DMF) through vigorous sonication to achieve a homogenous suspension.The DMF also plays as a surface surfactant to promote MWCT and CNF’sdisaggregation and uniform dispersion.

The buckypaper was prepared by using the vacuum filtration methodthrough a nylon membrane. After drying, a thin film layer was peeledfrom the filter membrane to produce a free-standing buckypaper as shownin FIG. 5B. Pt nanoparticles were electrochemically deposited onto thebuckypaper by using a base mixture solution of 10 mM H2PtCI₆, 0.1 MH₂SO₄, and 0.5 M ethylene glycol with N₂ bubbling and by applying squarecurrent pulses. The applied potential increased from 0.2 V to -0.25 V(versus the saturated calomel electrode) with a pulse width of 1 s and apulse duty cycle of 25%. The pulse was repeated until the desired Ptloading was reached. The Pt loading was determined by weighing the massdifference before and after deposition. The loading if Pt isproportional to numbers of current pulses as shown in FIG. 5C.

The Nafion electrolyte is grown by electrophoretic deposition (EPD), andthe resulting product is shown in FIG. 5D. Similar to electrochemicaldeposition, EPD is also an electron-charge exchange process that ensuresthat Nafion covers the Pt nanoparticles previously deposited byelectrochemical deposition.

The MEA was finalized by hot pressing together the cathode gas diffusionlayer (GDL), the Nafion membrane, the buckypaper anode catalyst layer,and the anode GDL with a micro porous layer. Additional Nafion solutionwas sprayed on the surface of catalyst layers before the hot press.

The invention has a three-dimensional electrode structure withfunctional gradient buckypaper, which was prepared by filtering theCNT/CNF mixture and the CNF suspension sequentially (FIG. 6A). Thebuckypaper was made with graded porosity with high porosity near the gasdiffusion layer interface, benefitting mass transfer, and low porosityat the electrode-membrane interface, allowing high Pt loading density atthis interface. Pt nanoparticles were deposited non-uniformly onbuckypaper using a pulsed electrodeposition technique. The concentrationof Pt particles decreased from the membrane side to the air side asshown in FIG. 6B. SEM images of the surfaces of the CNT/CNF sublayer andthe CNF sublayer are shown in FIG. 6C and FIG. 6D, respectively. FIGS. 7and 8 show the cell potential and the power density as a function ofcurrent density by using H₂/O₂ and H₂/Air, respectively. The Pt loadingsat the anode and the cathode were 0.05 and 0.25 mg/cm², respectively andthe operating conditions are denoted on the figures. The rated powerdensities are 1,500 and 800 mW/cm², the Pt utilizations are 0.167 and0.31 g_(Pt)/kW for H₂/O₂ and H₂/Air, respectively. FIG. 7 and FIG. 8were obtained from MEAs made with double layer buckypapers. FIG. 7 wasobtained with an MEA tested under oxygen flow in the cathode. FIG. 8 wasobtained from and MEA tested under air (about 20% oxygen) flow in thecathode

Buckypaper supported Pt with a loading of 0.4 mg/cm² was used as cathodecatalyst layer in the MEAs. The mass activity of Pt/BP catalyst was only0.05 A/mg which is half of the-state-of-the-art Pt/C catalyst due torelatively large Pt particles (4-6 nm) synthesized by electrochemicaldeposition.

In this invention, the nano- and micro-structure of the catalyst layerswere tailored in order to design an optimum gradient porosity thatfacilitates the electrochemical reactions and improves the cellperformance. The fabricated buckypapers with gradient structure usingthe filtration method demonstrated a significant improvement in thepower density of the PEMFCs. In order to achieve the optimummicrostructure, the porosity and surface area can be controlled byselecting CNTs and CNFs with different sizes and adjusting their ratiowith respect to each other. The PGM nanoparticles are electrodepositedon the most accessible sites in the buckypaper and are not be covered byeither CNTs or CNFs. The nanoparticle size is determined by thedeposition conditions such as time and current density. The particledensity is determined by the surface area (nonuniform distribution) anddensity of defect sites, which is pre-etched on the CNT surface. Thesecond structure is the binary alloy structure via electroplating. Bothdirect current and pulse current (PC) methods were explored, and theeffect of PC waveforms, including pulse-reverse, on the coating particlecomposition, morphologies, average size, size distribution, and moreimportantly, ORR efficiency, was systematically investigated. Theaverage particle size was found to be between 4-6 nm to ensure thedurability of catalytic electrodes. The Nafion electrolyte was grown byEPD. A library of EPD process parameters such as suspension solution,working voltage, and deposition time, which can be adjusted to achieve arange of deposited compositions and coating thicknesses.

One of the major costs in fabricating PEMFC systems for automotive andstationary power applications is the cost of the PGM cathode electrodecatalyst. The invention makes possible the production of a fuel cellthat is more affordable and more durable, because this invention cansignificantly improve Pt utilization by optimization of the triple phaseboundary condition. Therefore, the loading of total Pt can be reducedand the cost of MEAs as well as fuel cells will be lower. This inventionalso used buckypapers as the supporting material of Pt nanoparticles.The lifetime of the MEAs and fuel cells can also benefit from the goodresistance to chemical corrosion of CNTs due to the surfacegraphitization.

The invention as shown in the drawings and described in detail hereindisclose arrangements of elements of particular construction andconfiguration for illustrating preferred embodiments of structure andmethod of operation of the present invention. It is to be understoodhowever, that elements of different construction and configuration andother arrangements thereof, other than those illustrated and describedmay be employed in accordance with the spirit of the invention, and suchchanges, alternations and modifications as would occur to those skilledin the art are considered to be within the scope of this invention asbroadly defined in the appended claims. In addition, it is to beunderstood that the phraseology and terminology employed herein are forthe purpose of description and should not be regarded as limiting.

We claim:
 1. A method of making a catalyst layer of a membrane electrodeassembly (MEA) for a polymer electrolyte membrane fuel cell, comprisingthe steps of: preparing a porous buckypaper layer comprising at leastone selected from the group consisting of carbon nanofibers and carbonnanotubes; depositing platinum group metal nanoparticles in a liquidsolution on an outer surface of the buckypaper to create a platinumnanoparticle buckypaper; and, depositing a proton conducting electrolyteon the platinum nanoparticles by electrophoretic deposition to create aproton-conducting layer on an outer surface of the platinumnanoparticles; depositing an additional proton-conducting layer bycontacting the platinum nanoparticle buckypaper with a liquidproton-conducting composition in a solvent; drying the platinumnanoparticle buckypaper to remove the solvent.
 2. The method of claim 1,wherein the step of contacting the platinum nanoparticle buckypaper withliquid proton-conducting composition in the solvent comprises at leastone selected from the group consisting of the liquid drop method and theliquid dipping method.
 3. The method of claim 1, wherein theproton-conducting electrolyte comprises at least one selected from thegroup consisting of Nafion, polyvinylidene fluoride (PVDF)/Nafioncomposite, and Nafion/silica composite.
 4. The method of claim 1,wherein the proton-conducting layer is from 2-10 wt %, based on thetotal weight of the catalyst layer.
 5. The method of claim 1, whereinthe buckypaper has a porosity of from 50% to 90% before the depositionof the platinum group metal nanoparticles and the proton-conductinglayer.
 6. The method of claim 1, wherein the buckypaper layer has agraduated porosity, with the porosity being less on a side of thebuckypaper layer to abut a proton exchange membrane of the membraneelectrode assembly.
 7. The method of claim 6, wherein the porosity ofthe buckypaper layer is graduated from a maximum porosity difference of40% to a minimum porosity difference of 10%.
 8. The method of claim 1,wherein the platinum group metal nanoparticles are depositedelectrochemically.
 9. The method of claim 1, wherein the platinum groupmetal nanoparticles have a dimension of from 2 to 10 nm.
 10. The methodof claim 1, wherein the platinum group metal nanoparticles are from 30to 80% wt %, based on the total weight of the catalyst layer.
 11. Themethod of claim 1, wherein the buckypaper has less than 1% binder, basedon the total weight of the buckypaper layer.
 12. The method of claim 1,wherein the platinum group metal (PGM) nanoparticles comprise at leastone selected from the group consisting of platinum, platinum nickelalloy, platinum copper alloy, platinum cobalt alloy, platinum ironalloy, platinum iridium alloy, and platinum palladium alloy.
 13. Themethod of claim 1, wherein the platinum group metal nanoparticles arecore-shell structures including a platinum shell and a core comprisingat least one selected from the group consisting of nickel, copper,cobalt, iron, iridium, and palladium.
 14. The method of claim 1, whereina proton conductivity of the proton-conducting layer is from 0.01-0.2Siemens/cm.